func fib(n int) int {
    dp := make([]int, 2)
    if n <= 1 {return n}
    dp[0] = 0
    dp[1] = 1
    for i:=2; i<=n; i++ {
        sum := dp[0] + dp[1]
        dp[0] = dp[1]
        dp[1] = sum
        
    }
    return dp[1]
}